To comply with the current demand for high density recording, improvement of a magnetic recording medium is now under extensive investigation. The most promising medium that can be expected to increase in recording density is the so-called metal thin film type of medium in which a metal thin film is used as a magnetic layer.
Magnetic recording systems can be roughly divided into two types: longitudinal magnetic recording that has an axis of easy magnetization in the longitudinal direction of the magnetic layer surface (i.e., which is magnetized in the direction parallel to the magnetic layer surface), and perpendicular magnetic recording that has an axis of easy magnetization in the perpendicular direction of the magnetic layer surface (i.e., which is magnetized in the direction perpendicular to the magnetic layer surface). Various improvements in magnetic material compositions, layer constructions and film-forming methods depending on the respective recording systems have been proposed for a magnetic recording medium which has a metal thin film as the magnetic layer. Compositions known to comprise the thin film magnetic recording layer for the longitudinal magnetic recording include CoNi, CoNiCr, etc. When forming a thin film as the magnetic layer using the known composition according to the usual magnetron sputtering method, it is necessary to carry out the sputtering while heating a nonmagnetic substrate at a temperature range of from about 100.degree. to about 350.degree. C. in order to increase coercive force.
Compositions known to comprise the thin film magnetic recording layer of the perpendicular magnetic recording include CoCr, CoCrTa, etc. In the film formation according to the usual magnetron sputtering method, it is necessary to heat the nonmagnetic substrate at a temperature of not less than 100.degree. C. in order to obtain perpendicular magnetic anisotropy. In the case of the vacuum deposition method, heating at a much higher temperature is needed.
When heating the nonmagnetic substrate in the formation of the magnetic metal thin film, a film-forming apparatus is needed which is complicated in structure, and limits the type of the nonmagnetic substrate that can be used. For example, plastic materials such as a polyethylene terephthalate film are unsuitable for use.
Known materials that provide a high coercive force even in film formation at low temperatures include a CoPt-based longitudinal magnetic recording film and a CoPtBO-based perpendicular magnetic recording film (JP-A-2-74012, etc.) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). The Pt-based material, however, has a problem of being expensive for practical use.
A CoPd-based metal thin film is inexpensive as compared with the Pt-based material and can provide a high coercive force even in film formation at low temperatures, and is disclosed in, for example, JP-A-1-191318 and JP-A-2-30104. However, Pd is needed in a proportion of not less than 60% and, therefore, the CoPd-based material is still expensive. Moreover, the saturation magnetization Ms of the material is about 400 emu/ml, and thus the material does not have sufficiently satisfactory magnetic characteristics.
J. Appl. Phys., 52 (3), 2453 (1981) describes the magnetic anisotropy of a Co--Cr-based metal thin film as the magnetic film of the perpendicular magnetic recording. In this report, alloy thin films in which small amounts of Pd and Rh are added to the Co--Cr are disclosed but with only small improvements to the above cited problems.